As a positive-working photoresist composition, a composition containing an alkali-soluble resin and a naphthoquinonediazide compound as a photosensitive material is generally used. For example, such compositions are described in U.S. Pat. Nos. 3,666,473, 4,115,128, 4,173,470, etc., as "novolac-type phenol resin/naphthoquinonediazide substitution compound" and also, as the most typical positive-working photoresist composition, the examples of "a novolac resin composed of cresol-formaldehyde/trihydroxybenzophenone-1,2-naphthoquinonediazidosulfon ic acid ester" are described in L. F. Thompson, Introduction to Microlithography, No. 219, pages 112-121, published by ACS.
Since the novolac resin, as the binder, is soluble in an aqueous alkali solution without being swelled and since, in the case of using the image formed as the mask for etching, the novolac resin gives a particularly high durability to plasma etching, the novolac resin is particularly useful as a photoresist composition.
Also, the naphthoquinonediazide compound being used as the photosensitive material functions as a dissolution inhibitor lowering the alkali solubility of the novolac resin by itself, but is unique in that when the compound is decomposed by being irradiated with light, the compound forms an alkali-soluble material and apparently functions to increase the alkali-solubility of the novolac resin. Further, because of the large property change by light, the naphthoquinonediazide compound is particularly useful as the photosensitive material for a positive-working photoresist composition.
Hitherto, from such a view point, many positive-working photoresist compositions each containing the novolac resin and the naphthoquinonediazide series photosensitive material have been developed and practically used, and the compositions have obtained a sufficient result in line-width working to the extent of from about 1.5 .mu.m to 2 .mu.m.
However, an integrated circuit is increasing the integration degree and, in the production of a semiconductor device such as a very large scale integrated circuit (LSI), etc., working of a very fine pattern composed of a line width of not wider than 1 .mu.m has been required. In such a use, photoresist having a particularly high resolving power, a high pattern form- reproducing accuracy capable of accurately reproducing the form of a light-exposure mask, and a high sensitivity from the view point of a high productivity has been required. However, the conventional positive-working photoresists described above have not satisfied the foregoing requirements.